Field of the Invention
The present arrangement relates to communication cables. More particularly, the present arrangement relates to data communication cables using modified insulation.
Description of the Related Art
In the communication industry, one type of a common communication cable is the LAN (Local Area Network) cable, formed from four pairs of conductors. The conductor pairs are made from two wires twisted around one another, commonly referred to as a twisted pair. Typical high speed communication cables may include a number of shielded or unshielded twisted pairs enclosed by an outer jacket.
One problem that typically confronts the construction of such cables is signal interference or crosstalk that can occur between twisted pairs within the cable as well as with interference from other signal sources outside the cable, in particular with unshielded twisted pairs running in adjacent cables. In order to reduce the incidences of cross talk, the twisted pairs in unshielded data communication cables have different twist rates from one another so that a typical four pair LAN cable will have 4 pairs each with a different twist rate.
However, due to the different twist rates for addressing crosstalk, another cable construction obstacle arises referred to as skew. For example, for any given length of cable, the same signal sent along two adjacent twisted pairs with different twist rates will reach the end of the cable at different times. This occurs because the twisting of one pair at a shorter lay length (higher twist rate) than another pair within the same cable will necessarily result in the physical conductor path in the shorter lay length pair being longer than the conductor path of the pair(s) with the longer lay length (slow rate of twist). This resultant time difference is known as skew.
The property of skew is not only influenced by the physical length of the conductors in the various pairs. The insulation used on the pairs also affects the speed of signal propagation. This effect is a result of the communication signal passing in part through the insulation on the conductor pairs, slowing the propagation rates. Thus, in the longer (shorter lay length) pairs, the dielectric coupling of the signal to the insulation slows the propagation rates.
Moreover, each polymer used for insulation has its own dielectric constant. Certain polymers have low dielectric constants with a corresponding lesser effect on the signal speed. An example of such a polymer is FEP (Fluorinated Ethylene Propylene Copolymer). Other polymers such as Polypropylene have higher dielectric constants and thus exhibit a greater negative effect on the signal speed. This further exacerbates the skew problem.
The way the prior art has addressed the problem of skew is to increase the relative signal propagation velocity in the slower pairs by foaming the insulation used on those pairs. By foaming the insulation, the dielectric constant is reduced, thus allowing the signal in the slow pairs (pairs with shorter lay length) to be faster relative to the faster pair (pair with the longest lay length) reducing the overall signal velocity difference in the cable pairs and thus reducing skew.
However, the foaming process has a number of disadvantages; it is expensive, causes reduced manufacturing line speeds (slow extrusion), is difficult to control and ultimately yields high scrap rates. In addition, foamed insulation is easier to crush and thus may lead to the cables/pairs failing the necessary crush resistance testing. In fact, the foamed insulation may even overly compress/crush during twining (of the conductors into pairs). As a result, the insulation on the foamed pairs must be oversized to compensate. This increases the overall diameter of the cable which creates problems for the end user since they typically prefer smaller diameter cables.